Current steering digital-to-analog converter (DAC) is widely used in modern analog and mixed-signal circuit system especially for high sampling rate applications. It consists of an array of current sources, the number of array depends on DAC's decoding scheme either thermometer, binary or segmented. The DAC linearity is mainly determined by the matching of the current sources.
When designing a high speed DAC many tradeoffs have to be taken into account. These include the type of the DAC (current steering or a voltage output DAC), the number of bits and required speed. For high speed DACs usually a current steering converter is selected because of the speed advantages of this converter type with respect to the voltage output DAC. In most current steering DAC designs a combination of binary and thermometer code weighting is used. The thermometer code is used for the Most Significant Bits (MSBs), while binary code for the Least Significant Bits (LSBs) is used. This is called segmentation: 0% segmentation means a fully binary converter and 100% segmentation means a full thermometer code converter. The DAC designer has to choose the optimum amount of segmentation taking into account the physical problems associated with the segmentation. Current steering DACs with a low amount of segmentation have the advantage that they are simple. They only need a few current sources and switches. The disadvantage of low segmentation is the possible larger DNL compared to the converter converters with larger segmentation. Converters with larger segmentation result in a lower DNL, but the problems associated with the output conductance, capacitance and charge feed-through increase and also the area and power that is needed increases. In addition, due to the larger area required and the larger difference in output impedance between the lowest output code and the highest output code, the timing errors become more dominant.
The basic principle of current steering DACs is the summation of currents according to the input. In a binary current steering converter, the current sources are connected parallel to each other. These current sources are connected to switches; the switches connect the current source to the output node. The switches are controlled by the input code of the DAC. The output current of the DAC is therefore proportional to the input code word. The output node of the DAC is connected to a resistor. This resistor converts the output current of the DAC into a voltage.
FIG. 1 shows a conventional PFET current source and NFET current source. In the PFET current source, a thick oxide transistor or device 10 is connected to the voltage rail. A positive thick oxide device 12 switches the output of thick oxide device 10 (which is connected to VDD supply rail) to generate a positive current output and a negative thick oxide device 14 switches the output of the thick oxide device 10 to generate a negative current output. Similarly, for an NFET current source, a positive thick oxide device 22 switches the output of thick oxide device 20 to generate a positive current output and a negative thick oxide device 14 switches the output of thick oxide 20 (which is connected to ground) to generate a negative current output.
The technology process from chip manufacturers usually provides two kinds of transistor devices: thick-oxide and thin-oxide devices. Faster thin-oxide devices sometimes are called core devices and they are used mainly for logic circuitry. Thick-oxide device can operate at higher supply voltage and they are used in IO, interface and analog circuitry. For advanced process such as 65 nm or below, the operation supply voltage for core device is around 1.0 Volt and the supply voltage for thick-oxide device is either 2.5V or 3.3V. Operating thin-oxide devices on higher supply results in voltage stress, device breakdown and causes device reliability issue. The mismatch of devices is mainly due to the threshold voltage of devices. Since the thin-oxide has smaller threshold voltage, its matching characteristic is superior to thick-oxide devices. For IO supply powered DAC such as 3.3V, conventionally thick-oxide devices are required in the current source to prevent the voltage stress on the devices.